The invention is related to utilization of multi-user channel, where a single transmitting station sends data to several receiving stations within the same resource unit. If a transmitting station has several antennas, it is possible to separate the users only by using the spatial properties of the channel. This results to significantly enhanced system throughput since the users do not have to be separated in time, frequency or code domain.
Recently, multiple input, multiple output (MIMO) wireless technologies have been getting a lot of attention. MIMO technology has been suggested for use in several application areas, including future cellular communications, vehicle communications, wireless local and wide area networks, and military communications.
MIMO technology has the ability to significantly increase channel capacity and at the same improve link reliability. The advantages of MIMO technologies are achieved through the use of spatial diversity. Data in a MIMO system is transmitted over a plurality of transmit antennas to a plurality of receive antennas in the receiver terminals. When multiple signals are transmitted over a number of channels simultaneously, the signals between the plurality of transmit and receive antennas fluctuate and fade independently. The transmitted data may be encoded in the transmitter with a specific space-time code and by performing a corresponding processing at the receiver, the underlying data may be appropriately extracted. Possibilities of such combined preceding and equalizer design are currently intensively studied.
For the case where each user has only a single receive antenna, various precoding schemes are available in the literature. For the more general case with multiple receive antennas per user, some schemes have been recently proposed. Examples of such prior art document comprise Q. H. Spencer, A. L. Swindlehurst, and M. Haardt, “Zero-Forcing Methods for Downlink Spatial Multiplexing in Multiuser MIMO Channels,” IEEE Trans. Signal Processing, vol. 52, no. 2, pp. 461-471, February 2004, and V. Stankovic and M. Haardt, “Multi-User MIMO Downlink Precoding for users with multiple antennas,” in Proceedings of the 12th meeting of the Wireless World Research Forum (WWRF), Toronto, ON, Canada, November 2004. However, the techniques proposed so far rely on ad-hoc designs stemming from the single receive antenna case and are therefore not optimum in the minimum mean square error (MMSE) sense.